Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image foaming apparatuses may employ an induction heater to warm up the fixing device quickly to a predetermined fixing temperature with reduced energy consumption. For example, the induction heater is disposed opposite a fixing roller that presses against a pressing roller to form a fixing nip between the fixing roller and the pressing roller. As a recording medium bearing a toner image passes through the fixing nip, the fixing roller heated by the induction heater and the pressing roller apply heat and pressure to the recording medium, thus melting and fixing the toner image on the recording medium.
Specifically, the induction heater includes an exciting coil that generates a magnetic flux toward a conductive layer of the fixing roller. As the magnetic flux reaches the conductive layer of the fixing roller, the conductive layer generates an eddy current that heats the conductive layer throughout the entire width of the fixing roller in the axial direction thereof. However, if a small recording medium having a width smaller than the entire width of the fixing roller in the axial direction thereof is conveyed through the fixing nip, the lateral ends of the fixing roller in the axial direction thereof over which the small recording medium is not conveyed may be overheated because the small recording medium does not draw heat from the lateral ends of the fixing roller in the axial direction thereof
To address this circumstance, degaussing coils may be disposed between the exciting coil and the fixing roller in such a manner that the degaussing coils are disposed opposite the lateral ends of the fixing roller in the axial direction thereof, respectively, to offset the magnetic flux generated by the exciting coil toward the fixing roller, thus minimizing the magnetic flux that reaches the conductive layer of the fixing roller and therefore preventing overheating of the lateral ends of the fixing roller in the axial direction thereof For example, when the image forming apparatus receives a print job for forming a toner image on a small recording medium, the degaussing coils are turned on. Conversely, when the image forming apparatus receives a print job for forming a toner image on a large recording medium, the degaussing coils are turned off.
However, such configuration has a drawback in that the degaussing coils cannot be turned on and off while the exciting coil is turned on because serially-connected relays used to turn on and off the degaussing coils may be short-circuited and melted. To address this circumstance, it is necessary to turn off the exciting coil temporarily while the degaussing coils are turned on and off, generating variation in the temperature of the fixing roller in the direction of rotation of the fixing roller. Specifically, since the fixing roller rotates even while the exciting coil is turned off temporarily, a section of the fixing roller that passes through the induction heater while the exciting coil is turned off is not heated by the induction heater. Accordingly, the fixing roller has a heated section heated by the induction heater and a non-heated section not heated by the induction heater, resulting in temperature variation of the fixing roller in the direction of rotation of the fixing roller. Consequently, the fixing roller heats the toner image on the recording medium unevenly, thus forming a faulty toner image on the recording medium.